Strange sound from UFO footage in Singapore (Video)

Update: I took another round of check on the video. I think the sound is from the birds. I tried to align the sound in normal speed and the sound in slow motion. Seems to align.. So, no mystery here.. thanks for coming here and sorry to waste your time

Over the short span of a 23 seconds, when I played it 8 times slower, I can heard strange sound and it repeats 5 times in the clip. I am unable to hear it with original speed. It is only when the video is in slow motion, then I can heard it distinctively. Check out this video.

In the video, the first 20 odd seconds is the original video. After that, I slowed the video down by 8 times and amplified the sound by 14db using Audacity. Along the way, some sound are clipped (too loud).

If you know what the sound is, please let me know. Or if you have seen similar clip, let me know too. Thanks.